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<div class="section">
<div class="titlepage"><div><div>
<h2 class="title">
<a id="complex-core-fields"></a>Complex Core Field Types<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/elasticsearch-definitive-guide/edit/2.x/052_Mapping_Analysis/50_Complex_datatypes.asciidoc">edit</a>
</h2>
</div></div></div>
<p>Besides the simple scalar datatypes that we have mentioned, JSON also
has <code class="literal">null</code> values, arrays, and objects, all of which are supported by
Elasticsearch.</p>
<div class="section">
<div class="titlepage"><div><div>
<h3 class="title">
<a id="_multivalue_fields"></a>Multivalue Fields<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/elasticsearch-definitive-guide/edit/2.x/052_Mapping_Analysis/50_Complex_datatypes.asciidoc">edit</a>
</h3>
</div></div></div>
<p>It is quite possible that we want our <code class="literal">tag</code> field to contain more
than one tag. Instead of a single string, we could index an array of tags:</p>
<div class="pre_wrapper lang-js">
<pre class="programlisting prettyprint lang-js">{ "tag": [ "search", "nosql" ]}</pre>
</div>
<p>There is no special mapping required for arrays. Any field can contain zero,
one, or more values, in the same way as a full-text field is analyzed to
produce multiple terms.</p>
<p>By implication, this means that <em>all the values of an array must be
of the same datatype</em>.  You can’t mix dates with strings. If you create
a new field by indexing an array, Elasticsearch will use the
datatype of the first value in the array to determine the <code class="literal">type</code> of the
new field.</p>
<div class="note admon">
<div class="icon"></div>
<div class="admon_content">
<p>When you get a document back from Elasticsearch, any arrays will be in the
same order as when you indexed the document.  The <code class="literal">_source</code> field that you get
back contains exactly the same JSON document that you indexed.</p>
<p>However, arrays are <em>indexed</em>—made searchable—​as multivalue fields,
which are unordered.  At search time, you can’t refer to “the first element”
or “the last element.”  Rather, think of an array as a <em>bag of values</em>.
</p>
</div>
</div>
</div>

<div class="section">
<div class="titlepage"><div><div>
<h3 class="title">
<a id="_empty_fields"></a>Empty Fields<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/elasticsearch-definitive-guide/edit/2.x/052_Mapping_Analysis/50_Complex_datatypes.asciidoc">edit</a>
</h3>
</div></div></div>
<p>Arrays can, of course, be empty. This is the equivalent of having zero
values. In fact, there is no way of storing a <code class="literal">null</code> value in Lucene, so
a field with a <code class="literal">null</code> value is also considered to be an empty
field.</p>
<p>These three fields would all be considered to be empty, and would not be
indexed:</p>
<div class="pre_wrapper lang-js">
<pre class="programlisting prettyprint lang-js">"null_value":               null,
"empty_array":              [],
"array_with_null_value":    [ null ]</pre>
</div>
</div>

<div class="section">
<div class="titlepage"><div><div>
<h3 class="title">
<a id="inner-objects"></a>Multilevel Objects<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/elasticsearch-definitive-guide/edit/2.x/052_Mapping_Analysis/50_Complex_datatypes.asciidoc">edit</a>
</h3>
</div></div></div>
<p>The last native JSON datatype that we need to discuss is the <em>object</em> — known in other languages as a hash, hashmap, dictionary or
associative array.</p>
<p><em>Inner objects</em> are often used to embed one entity or object inside
another. For instance, instead of having fields called <code class="literal">user_name</code>
and <code class="literal">user_id</code> inside our <code class="literal">tweet</code> document, we could write it as follows:</p>
<div class="pre_wrapper lang-js">
<pre class="programlisting prettyprint lang-js">{
    "tweet":            "Elasticsearch is very flexible",
    "user": {
        "id":           "@johnsmith",
        "gender":       "male",
        "age":          26,
        "name": {
            "full":     "John Smith",
            "first":    "John",
            "last":     "Smith"
        }
    }
}</pre>
</div>
</div>

<div class="section">
<div class="titlepage"><div><div>
<h3 class="title">
<a id="_mapping_for_inner_objects"></a>Mapping for Inner Objects<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/elasticsearch-definitive-guide/edit/2.x/052_Mapping_Analysis/50_Complex_datatypes.asciidoc">edit</a>
</h3>
</div></div></div>
<p>Elasticsearch will detect new object fields dynamically and map them as
type <code class="literal">object</code>, with each inner field listed under <code class="literal">properties</code>:</p>
<div class="pre_wrapper lang-js">
<pre class="programlisting prettyprint lang-js">{
  "gb": {
    "tweet": { <a id="CO21-1"></a><i class="conum" data-value="1"></i>
      "properties": {
        "tweet":            { "type": "string" },
        "user": { <a id="CO21-2"></a><i class="conum" data-value="2"></i>
          "type":             "object",
          "properties": {
            "id":           { "type": "string" },
            "gender":       { "type": "string" },
            "age":          { "type": "long"   },
            "name":   { <a id="CO21-3"></a><i class="conum" data-value="2"></i>
              "type":         "object",
              "properties": {
                "full":     { "type": "string" },
                "first":    { "type": "string" },
                "last":     { "type": "string" }
              }
            }
          }
        }
      }
    }
  }
}</pre>
</div>
<div class="calloutlist">
<table border="0" summary="Callout list">
<tr>
<td align="left" valign="top" width="5%">
<p><a href="#CO21-1"><i class="conum" data-value="1"></i></a></p>
</td>
<td align="left" valign="top">
<p>Root object</p>
</td>
</tr>
<tr>
<td align="left" valign="top" width="5%">
<p><a href="#CO21-2"><i class="conum" data-value="2"></i></a><a href="#CO21-3"></a></p>
</td>
<td align="left" valign="top">
<p>Inner objects</p>
</td>
</tr>
</table>
</div>
<p>The mapping for the <code class="literal">user</code> and <code class="literal">name</code> fields has a similar structure
to the mapping for the <code class="literal">tweet</code> type itself.  In fact, the <code class="literal">type</code> mapping
is just a special type of <code class="literal">object</code> mapping, which we refer to as the
<em>root object</em>.  It is just the same as any other object, except that it has
some special top-level fields for document metadata, such as <code class="literal">_source</code>,
and the <code class="literal">_all</code> field.</p>
</div>

<div class="section">
<div class="titlepage"><div><div>
<h3 class="title">
<a id="_how_inner_objects_are_indexed"></a>How Inner Objects are Indexed<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/elasticsearch-definitive-guide/edit/2.x/052_Mapping_Analysis/50_Complex_datatypes.asciidoc">edit</a>
</h3>
</div></div></div>
<p>Lucene doesn’t understand inner objects. A Lucene document consists of a flat
list of key-value pairs.  In order for Elasticsearch to index inner objects
usefully, it converts our document into something like this:</p>
<div class="pre_wrapper lang-js">
<pre class="programlisting prettyprint lang-js">{
    "tweet":            [elasticsearch, flexible, very],
    "user.id":          [@johnsmith],
    "user.gender":      [male],
    "user.age":         [26],
    "user.name.full":   [john, smith],
    "user.name.first":  [john],
    "user.name.last":   [smith]
}</pre>
</div>
<p><em>Inner fields</em> can be referred to by name (for example, <code class="literal">first</code>). To distinguish
between two fields that have the same name, we can use the full <em>path</em> (for example, <code class="literal">user.name.first</code>) or even the <code class="literal">type</code> name plus
the path (<code class="literal">tweet.user.name.first</code>).</p>
<div class="note admon">
<div class="icon"></div>
<div class="admon_content">
<p>In the preceding simple flattened document, there is no field called <code class="literal">user</code>
and no field called <code class="literal">user.name</code>.  Lucene indexes only scalar or simple values,
not complex data structures.</p>
</div>
</div>
</div>

<div class="section">
<div class="titlepage"><div><div>
<h3 class="title">
<a id="object-arrays"></a>Arrays of Inner Objects<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/elasticsearch-definitive-guide/edit/2.x/052_Mapping_Analysis/50_Complex_datatypes.asciidoc">edit</a>
</h3>
</div></div></div>
<p>Finally, consider how an array containing inner objects would be indexed.
Let’s say we have a <code class="literal">followers</code> array that looks like this:</p>
<div class="pre_wrapper lang-js">
<pre class="programlisting prettyprint lang-js">{
    "followers": [
        { "age": 35, "name": "Mary White"},
        { "age": 26, "name": "Alex Jones"},
        { "age": 19, "name": "Lisa Smith"}
    ]
}</pre>
</div>
<p>This document will be flattened as we described previously, but the result will
look like this:</p>
<div class="pre_wrapper lang-js">
<pre class="programlisting prettyprint lang-js">{
    "followers.age":    [19, 26, 35],
    "followers.name":   [alex, jones, lisa, smith, mary, white]
}</pre>
</div>
<p>The correlation between <code class="literal">{age: 35}</code> and <code class="literal">{name: Mary White}</code> has been lost as
each multivalue field is just a bag of values, not an ordered array.  This is
sufficient for us to ask, "Is there a follower who is 26 years old?"</p>
<p>But we can’t get an accurate answer to this: "Is there a follower who is 26 years old <em>and who is called Alex Jones</em>?"</p>
<p>Correlated inner objects, which are able to answer queries like these,
are called <em>nested</em> objects, and we cover them later, in
<a class="xref" href="nested-objects.html" title="Nested Objects"><em>Nested Objects</em></a>.</p>
</div>

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